Superconducting homogeneous high field magnetic coil

ABSTRACT

The invention concerns a superconducting high field magnetic coil for the production of magnetic field with one or more approximately rotationally symmetric hollow cylindrically shaped winding regions made from layers of windings of a superconducting wire through which current is flowing whereby in axial end regions of at least one winding region, radial components of the magnetic field are present which lead to axial Lorentz forces as well as to the accumulation of axial pressures in the winding layers. According to the invention, in the axial end regions of at least one winding region, the current density in at least one winding layer (S3, S4) is reduced relative to the normal current density present in the region bordering on the end region so that, thereby, the maximum Lorentz force occurring in these winding layers as well as the maximum occurring axial pressure is maintained within permissible limits.

The invention concerns a superconducting homogeneous high-field magneticcoil which is utilized for the production of a static magnetic field incombination with a nuclear magnetic resonance spectrometer.

The invention is based upon a multi-layer coil which is known in the artfrom the Journal of Physics E. Scientific Instruments, 1972, Pages 944through 946. In the coil which is known in the art the current densityin the middle lengthwise region is reduced through the addition of apassive wire winding in order to produce a homogeneous magnetic field.

Known in the art from patent abstracts of Japan Vol. 10 No. 233 (J-P-A.6 165 4 11) is the reduction of the average current density in the endregions through the addition of a passively wound wires. Thispublication discloses, however, a one layer coil and the purpose lies inthe limitation of the local increase of the magnetic fields in the endportion of the coil in order to prevent the critical field strength ofthe superconductor from being exceeded. There is no suggestion that thisconfiguration should be utilized in order to reduce the pressure withinthe coil produced in consequence of the Lorentz forces. The problem ofthe field increase with extremely thin one-layer solenoid coils istreated, a problem which in principle does not occur in multi-layeredcoils. The accumulated pressure is, in this case, no problem. On theother hand, in multi-layer high field magnet coils which are the objectof the invention, the field strength in and of itself is non-critical.

In order to produce magnetic fields of high magnetic field strength(many Tesla), among others, superconducting magnet coils are utilizedwhich are constructed from winding layers of a superconductable wire andexhibit approximately rotationally symmetric hollow cylinder shapedwinding cross sections. In order to realize the superconductingcapability of the wire material, by way of example Nb₃ Sn with copperstabilization, at the necessary current strengths and magnetic fieldobtaining in the vicinity of the wire, the magnetic coils operate withincryostats which, in the example mentioned, can be cooled by means ofliquid helium to a temperature of 4.2 K. In addition to thesuperconducting type A15 which is suitable for the highest fields and towhich Nb₃ Sn belongs, most recently, other superconducting materialshave become known in the art with which a utilization in a magnetconstruction well above 4.2 K, possibly at the temperature of liquidnitrogen, appear to be possible in the near future.

Due to the radial component of the magnetic fields produced by means ofthe magnet coil, axial pressures due to Lorentz forces occur in thewindings. The axial Lorentz forces increase with increasing radialcomponent of the magnetic field, that is to say, they are largest in theregion of the two coil ends. The resulting axial pressures, therebyaccumulate towards the coils' center, so that the maximum axial pressureoccurs in the coil center. These pressures, with high field magnets,obtain a value on the order of 50 Megapascals at the center of the coil(P. Turowski et al., 9th International Conference on Magnet Technology,p. 394, 1985) and can, when charging up the magnet, in particular inplaces in which the winding is not homogeneous, cause movement of theconductors which, in consequence of the heat generated by said movement,can cause a transition of the wire into the normally conducting state.Since the normally conducting region, despite low resistance copperstabilization, in consequence of the electrical resistance, continues toheat up the entire magnet coil quickly becomes normally conducting.

To produce the highest magnetic fields, the high field magnet coilsknown in the art are also operated at 1.8 K in order to achieve a highercurrent carrying capability of the conductors (see by way of example P.Turowski, Th. Schneider in 2nd High Field Conference Leuven 1988).

Superconducting high field magnetic coils which are utilized for theproduction of very homogeneous magnetic fields in the inside of thehollow cylinder, have, in contrast to inhomogeneous magnets, a largercoil length and are equipped with additional coils which compensate forthose magnetic field components of the high field magnet coils whichmost strongly determine the dependence of the magnetic field strength onthe axial position. Such additional coils are often introducedcompletely outside of the magnet and usually, in fact, are wound on aseparate coil body (see for example J. E. C. Williams et al., IEEETransactions on Magnetics, Vol 25, p. 1767). These additional magnetcoils can, however, also be realized in that regions are introduced inthe windings themselves in which the current density is reduced withrespect to that in the surrounding winding regions (see here by way ofexample L. Cesnak and D. Kabat, Journal of Physics E. ScientificInstruments 944, 1972). Such a region has the effect of a additionalcoil of negative current density. This method of homogenizing has theadvantage that the additional coil does not directly require additionalconstruction room and that the homogenizing region is easily introducedmore radially inward in the coil, as a result of which its effectivityis increased. The field region which is to be homogenized is located,thereby, at the coil center since the magnetic field here runs axiallyand the dependence of the magnetic field strength on the axial length isminimum. Thereby the regions of reduced current strength for thehomogenization of the magnetic field are also arranged in the coilcenter.

Superconducting high field magnets for the production of a veryhomogeneous magnetic field of the kind mentioned, as by way of exampleused in NMR spectroscopy, have accordingly, on the one hand, thedisadvantage that due to the larger necessary coil length the maximumaxial pressure is particularly large (the axial pressure accumulatesnamely in each winding layer over the individual windings from the coilends towards the coil middle so that its magnitude increases with thecoil length) and, on the other hand, the disadvantage that through theintroduction of the region of reduced current density, border surfacesresult between the regions of normal current density and thehomogenizing regions of reduced or vanishing current density. Theallowable axial pressures on these border surfaces are substantiallyless than in homogeneous layers of the winding. In consequence theachievable magnetic field strength in the superconducting magnet coilsfor the production of homogeneous magnetic fields is less than ofnon-homogenized magnetic coils. The magnetic field strengths which areachievable in superconducting high-field magnets known in the art withhomogenized magnetic fields lying in the region of 14 Tesla whereas withnon-homogenized magnetic coils, magnetic field strengths up to 20 Teslaare achievable. In a homogenized magnet for use in an NMR spectrometer,the basic homogeneity is typically good enough so that a homogeneity ofΔB/Bo<10 ppm and, in combination with a superconducting shim system, ahomogeneity of ΔB/Bo<1 ppm is achieved over an axial region of length>30mm. The magnetic field strengths which are currently achieved in NMRspectroscopy with this type of magnet vary with the coil type. Thereby,in a "standard bore" high field magnet with a room temperature bore of51.5 mm diameter, the present maximum achievable magnetic field strengthis 14.1 T. In a "wide bore" magnet with 89 mm room temperature bore,this value falls to 11.7 T, and in a "super wide bore" magnet with 150mm diameter room temperature bore, to 7 T. Depending on the constructiontype, such high field magnets are associated with varying expectationsfor the maximum achievable field range.

SUMMARY OF THE INVENTION

It is intended, in the context of the present application that the term"through which current is flowing only series",is meant to clearly statethat the superconducting wire, which is composed of a superconductablematerial and a normally conducting material never exhibits branches. Theexistence of branches result in an undefined current path so that coilswith this type of branching are not suitable for the production ofhomogeneous magnetic fields since into which of the respective availablepaths the entire current will flow into or from and in what ratio saidcurrent will distribute itself within the two paths is indefinite.

The underlying purpose of the invention is to, in magnets which are tobe homogenized, so control the large forces which occur in consequenceof the required increased construction lengths in such a way that astable superconduction is effected in the coil. This control of theforces is achieved in that a redirection of the forces from theoverstressed coil regions into lower stressed regions is effected. Withhomogeneous magnets, this introduces the possibility of achieving alarger field strength than that which has up to now been possible. Onthe other hand, the below mentioned method of force redistribution alsooffers the possibility of increasing the currently maximum controllablefield strength of superconducting magnets which have not beenhomogenized. The stated force or pressure redistribution is to beunderstood in that the coil in accordance with the invention, comparedto the coil known in the art, exhibits a pressure load which is lower insome portions of the coil and an increased pressure load in otherportions of the coil. Clearly, the coil in accordance with the inventionmust be calculated thoroughly.

The superconducting high field magnetic coil in accordance with theinvention with the features of the characterizing part of the main claimhas the advantage that the axial pressure load in each of the initiallyoverloaded winding layers can be reduced below the empiricallydetermined allowable upper limit. The axial pressure load of the windinglayer is given approximately by the sum over the contributions of theindividual windings of this layer. The size of the contribution of eachindividual winding thereby depends on the current density in theconductor as well as on the radial magnetic field strength at thelocation of the winding.

With the invention the current density is reduced "in many windinglayers". This implies that the superconducting wire exhibits a constantheight (measured in the radial direction of the coil), and whenutilizing a round wire, a constant diameter, since otherwise it wouldnot be possible to produce a plurality of winding layers, whereby eachof same must exhibit a uniform thickness.

Since the contributions to the accumulated axial pressure are largest inthe border region of the coil in consequence of the larger radialcomponent of the magnetic field present there, the configuration of theregions of reduced current density is particularly effective at the coilend regions. Towards this end it is not absolutely necessary that theregions of reduced current density extend to the end of the coil. Theconfiguration of the regions of reduced current density in the coilmiddle, on the other hand, would result in no substantial reduction inthe axial pressure load.

In the invention the expression "current density" is not to beinterpreted as "current density in superconductable material" since amere increase in the cross sectional area of the superconductablematerial at the expense of the cross sectional area of the copper wouldnot lead to the desired goal of the invention. Rather "current densityin at least one winding layer" refers to the average current density ina surface element of the winding cross section, which by way of exampleincludes at least two wire cross sections (so that the additionalpassive wire winding which is present under certain circumstances isalso included).

With the invention the current density in a location which is differentfrom the one in the above mentioned reference, namely the end region ofa coil winding, is reduced in many layers in such a way as to reduce theLorentz forces acting upon the superconducting wire. The measures inaccordance with the invention can be applied to partial windings whichexhibit the normal current density in their middle region as by way ofexample is the case with the partial winding S4 of the embodiment. Theinvention can however also be applied in a particularly advantageousfashion to such partial windings with which as in the above mentionedreference, a reduced current density is present in the middle regionwhich by way of example is produced through the addition of passive wirewinding through which no current is flowing and which is particularlysensitive to an overly strong pressure.

With appropriate application of the superconducting high field magnetwith a magnet coil in accordance with the invention it is possible toproduce higher homogeneous magnetic fields than those which were up tonow possible. This is possible in that, at the locations at which theallowable limit of the axial pressure load would have been exceeded, alocal pressure reduction is carried out.

In general it turns out, through the introduction of regions of reducedcurrent density in accordance with the invention, there is thepossibility that the magnet winding can be specifically operated over awide radial region at the allowable load for each radius, that is tosay, overloaded winding layers can be relieved and underloaded layersloaded more heavily since the introduction of regions of reduced currentdensity in winding layers leads to an increase in the axial pressureload in winding layers without such a region. This is true since thewindings which are missing in the regions with reduced current densitymust be introduced into other locations.

In an advantageous embodiment of the invention, the regions of reducedcurrent density are arranged symmetrically with respect to a plane whichruns perpendicular to the lengthwise axis of the coil at the middle ofthe coil. As was already discussed above, the contribution of individualwindings of a winding layer to the axial pressure load is the larger thecloser the winding is to the end of the coil so that a reduction is themore effective the further outside it is arranged in the coil. A maximumreduction of the current density at the coil end to zero is equivalentto a reduction in length of the winding in the corresponding layer.

In an advantageous embodiment of the invention one or more additionalregions of reduced current density are provided for in the vicinity ofthe coil middle. These homogenized regions of reduced current density inth coil middle are, when the pressure load there is initially largerthan the allowable maximum value, load reduced through the introductionof regions of reduced current density in regions of the coil ends in thesame radial location (namely position) and extension as in thehomogenizing regions. This has the advantage that in the inside of thehollow cylinder surrounded by the magnet coil it is possible to producea homogeneous magnetic field without the danger of destroying thesuperconductivity due to the reduced axial pressure loadability of theborder surfaces between the homogenizing region and the more denselywound region.

In an advantageous embodiment of the invention, the transition in theradial direction from a region of reduced current density to a region ofnormal current density takes place in a stepwise fashion. In this mannerone achieves the advantage that the substantial reduction of the axialpressure is tapered off in a stepwise fashion.

In a further advantageous embodiment of the invention the transition andthe radial direction from a region of reduced current density to aregion of normal current density takes place quasi-continuously withsteps which are given by the wire thickness. This has likewise theadvantage that the substantial reduction of the axial pressure is slowlytapered-off.

In further advantageous embodiments of the invention the stepwise orquasi-continuous transition in the radial direction between a region ofreduced current density and a region of normal current densitytranspires through appropriate changes in the axial extension of theregion of reduced current density and/or through an appropriate changein the contribution of the negative current density from winding layerto winding layer in the transition region.

Further features and advantages of the invention are manifested in thefollowing description of an embodiment by means of the drawing whichshow particular details of importance to the invention as well as fromthe claims. The individual features can be used alone or collectively inarbitrary combination in an embodiment of the invention. Shown are:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a lengthwise cross section of a superconducting high fieldmagnetic coil in accordance with the invention with four windingsections in an extremely schematic representation,

FIG. 2 a graphical representation of the axial pressure load in thiscoil,

FIGS. 3a and 3b two examples of the transition from normal currentdensity to reduced current density.

DETAILED DESCRIPTION

The superconducting high field magnetic coil which is represented in anextremely simplified fashion in FIG. 1 consists of four winding sectionsof which only the respective cross sectional plane contours are shown.The coil shape is given by the rotation of the figure section about therotational axis X which simultaneously corresponds to the lengthwiseaxis of the coil. A second axis is shown perpendicular to the axis Xupon which the radial separation R from the lengthwise axis of the coilin centimeters is recorded. The four coil winding sections S1 to S4 areeffected in differing fashions taking into consideration thesuperconducting capability of the material. While the two outermostwinding sections S3 and S4 are constructed from windings withniob-titan-wire with stabilizing copper, the second innermost windingsection S2 utilizes a binary A15 superconductor, and the innermostwinding cross section S1 a tertiary A15 superconductor, each with astabilizing copper core. The entire coil is realized in a fashionsymmetric with respect to a plane which runs perpendicular to thelengthwise coil axis X at the middle of the coil. The radius of thehollow cylinder surrounded by the magnet coil assumes a value of ca. 3.6cm. The innermost winding section S1 has an axial length of ca. 38.6 cmand a radial thickness of ca. 2.2 cm. The second innermost windingsection S2 has a separation of approximately 0.4 cm with respect to thefirst winding cross section S1, an axial length of approximately 44 cmand a radial extent of ca. 1.8 cm. The second outermost winding S3 has aseparation of 0.5 cm with respect to the second innermost winding S2, anaxial length of ca. 54 cm, a radial thickness of ca. 2.5 cm, andexhibits 32 winding layers. A rectangular NbTi conductor with 54filaments and 57.4% stabilizing copper is utilized, the dimensions ofwhich are 0.78×1.24 mm². The outermost winding section S4 has aseparation with respect to the second outermost S3 of 0.5 cm, an axiallength of ca. 56 cm, a radial extension of about 1.7 cm and consists of22 winding layers. A rectangular NbTi conductor with only 36 filamentsand a copper portion increased to 80% is utilized. The dimensions are0.78×1.24 mm². The innermost winding section S1 and the second innermostwinding section S2 are homogeneously wound through. The second outermostwinding cross section S3 exhibits a region symmetrically arranged aboutthe middle axis of the coil with a current density reduced to 50% whichstarts in the first winding layer and extends through to the 18thwinding layer and exhibits an axial extension of ca. 21.5 cm. Thisregion of reduced current density serves to homogenize the magnet andcauses the relative field change in an axial region of ±30 mm about thecoil center to be smaller than 10 ppm. Likewise symmetric to the coilmiddle plane and also starting in the first winding layers, a furtherregion of reduced current density is provided for in each of the twocoil end regions which extends up to 22 winding layers. The axial extentof each of these two regions assumes a value from the first through tothe 18th winding layer of ca. 6 cm and decreases from the 19th windinglayer in a plurality of steps of varying length (lengths from 2.25 cm,1.25 cm, 0.75 cm, 0.75 cm, 1 cm). (The numbers which are written on theleft end region of the winding S3 refer to the sum of the correspondinglength entries of both coil end regions).

The outermost winding section S4 exhibits for its part, in both of thecoil end regions, two regions which are symmetric with respect to themiddle plane of the coil which have 80% reduced current density andwhich extend through the layers 1 through 6. A step-down along thelength for transition to the full winding length is not necessary heredue to the only small change in the axial pressure.

The superconducting high field magnetic coil which is representedexhibits, at an operating current of 187.4 A an induction flux densityof 14.1 Tesla and has an inductance of 43.8 Henry. The contained energyassumes a value of 770 kJ. The high field magnetic coil is operated insuperconducting short circuit and has a field change (drift) versus timeof less than 10⁻⁷ per hour. The axial homogeneity region of the magnetcoil in which the induction flux density changes by less than 10 ppm,has a length≧60 mm and thereby satisfies the fundamental requirementsusually given for magnets for utilization in NMR spectroscopy.

FIG. 2 shows a graphical representation of the axial pressure load p inthe 4 winding sections, with the axial pressure load in Megapascalsbeing plotted versus the winding layer characterizing coil radius R. Inboth of the inner winding sections, the axial pressure is still quitesmall. In contrast, the axial pressure load in both of the outer windingsections is quite high. Here it is only possible to introduce a regionof reduced current density in the coil center if the axial pressure loadin the relevant winding layers is reduced. This transpires in accordancewith the invention with the additional introduction of regions ofreduced current density in both of the coil end regions. These regionsare extended in steps onto the neighbouring winding layers in order tostepwise taper-off the substantial reduction in the axial pressure.Furthermore, a reduction in the maximal occurring axial pressure isthereby achieved in this winding section. In the inner layers of theoutermost winding section S4 axial pressure would initially exceed avalue of 50 MPa. Through the introduction of the regions of reducedcurrent density in the inner winding layers in the end region of thecoil it is possible to reduce the maximum pressure load again to lessthan 50 MPa. In FIG. 2 the pressure load in the individual windinglayers without compensating measures is represented as a dashed linewhereas the solid line represents the pressure load after introductionof the compensating regions at the coil ends. One notices that not onlythe critical winding layers are load reduced but also that the pressureload in other uncritical winding layers is redistributed.

FIG. 3a shows a broken-off cross section of two layers of the winding S3in the left transition region to a region of reduced current density.The superconductable wire 20 is rectangular wire with the abovementioned dimensions. The superconducting material 22 is surrounded by ajacket 24 of copper. In the right portion of FIG. 3a, the normal averagecurrent density occurs during operation. Only half as large a currentdensity occurs in the left portion since a so-called blind wire 26which, at least for superconducting operation of the configuration, hasno current flowing through it, is wound in so that the separation frommiddle to middle between the individual windings of the wire 20 isdoubled. The same configuration can be applied when utilizing roundwire. The advantage of the configuration according to FIG. 3a is thatthe wire can be wound within each winding layer without any solderingjoints.

The right side of FIG. 3b corresponds with FIG. 3a. In the left part ofthis example, the superconducting wire 20' which here also containscopper and superconducting material goes over into a wire 20'' which istwice as wide as the wire 20', whereby however the cross section of thesuperconducting material is equally large in the wires 20' and 20''.

It is possible by way of example to utilize the following materials forthe winding sections S1 and S2: superconducting materials NbTi and/or anA15 superconductor by way of example Nb₃ Sn and/or Nb₃ SnTa or Nb₃ SnTiand/or YBa₂ Cu₃ O_(x) or Bi₂ Sr₂ CaCu₂ O₈ or Tl₂ Ba₂ CaCu₂ O₈ or Tl₂ Ba₂Ca₂ Cu₃ O₁₀ or another oxide material combination can be utilized.

The transition from a region of reduced current density into a region ofnormal current density must not, as shown in FIG. 1, occur abruptly orthrough a change in the axial or radial extent of the region, rather itcan also be achieved quasi-continuously through a changing of thethickness (in the lengthwise direction of the coil) or the number ofadditional passive wire windings (as described, by way of example, by L.Cesnak and D. Kabat in Journal of Physics E: Scientific Instruments 944,1972). These can but must not be the same cross section as thesuperconducting wire, or can also exhibit a rectangular shape.

What is claimed is:
 1. Superconducting high field magnetic coil for theproduction of a homogeneous strong magnetic field, the coil comprisingat least one approximately rotationally symmetric hollow cylindricallyshaped winding section having a plurality of winding layers of asuperconductable wire interconnected in order that current flows only inseries therethrough and wherein radial components of the magnetic fieldare present in an axial end region of said at least one winding section,causing axial Lorentz forces as well as an accumulation of axialpressure in the winding layers, said plurality of winding layers eachhave at least two regions of differing average winding layer crosssectional surface current density, with both normal current density andreduced current density, and in the vicinity of a coil middle, a regionof reduced current density extending over a plurality of winding layersin order to homogenize the magnetic field within a homogenizing volumenear a coil center, the plurality of winding layers being interconnectedin order that, in operation of the magnetic coil, a reduction in bothaxial end regions of at least one winding section of the current densityin each of a plurality of winding layers relative to a current densityin regions of the normal current density bordering the end regions, andmaximum axial pressures, acting on the wire, due to Lorentz forces inthe plurality of winding layers are held within allowable limits, saidwinding sections being configured so that in the end regions the currentdensity is reduced by increasing separation between neighboring windingsof the superconducting wire through the addition of passive wirewindings.
 2. Superconducting high field magnetic coil for the productionof a homogeneous strong magnetic field, the coil comprising at least oneapproximately rotationally symmetric hollow cylindrically shaped windingsection having a plurality of winding layers of a superconductable wireinterconnected in order that current flows only in series therethroughand wherein radial components of the magnetic field are present in anaxial end region of said at least one winding section, causing axialLorentz forces as well as an accumulation of axial pressure in thewinding layers, said plurality of winding layers each have at least tworegions of differing average winding layer cross sectional surfacecurrent density, with both normal current density and reduced currentdensity, and in the vicinity of a coil middle, a region of reducedcurrent density extending over a plurality of winding layers in order tohomogenize the magnetic field within a homogenizing volume near a coilcenter, the plurality of winding layers being interconnected in orderthat, in operation of the magnetic coil, a reduction in both axial endregions of at least one winding section of the current density in eachof a plurality of winding layers relative to a current density inregions of the normal current density bordering the end regions, andmaximum axial pressures, acting on the wire, due to Lorentz forces inthe plurality of winding layers are held within allowable limits, saidwinding sections being configured so that in the both axial end regionsof at least one winding section, the current density in that windinglayer with the region of reduced current density serving for thehomogenization is reduced to that in the normal current density of theregion bordering on the end region, and that, in remaining windinglayers of the axial end region, a transition to the normal currentdensity is effected.
 3. Superconducting high field magnetic coil for theproduction of a homogeneous strong magnetic field, the coil comprisingat least one approximately rotationally symmetric hollow cylindricallyshaped winding section having a plurality of winding layers of asuperconductable wire interconnected in order that current flows only inseries therethrough and wherein radial components of the magnetic fieldare present in an axial end region of said at least one winding section,causing axial Lorentz forces as well as an accumulation of axialpressure in the winding layers, said plurality of winding layers eachhave at least two regions of differing average winding layer crosssectional surface current density, with both normal current density andreduced current density, and in the vicinity of a coil middle, a regionof reduced current density extending over a plurality of winding layersin order to homogenize the magnetic field within a homogenizing volumenear a coil center, the plurality of winding layers being interconnectedin order that, in operation of the magnetic coil, a reduction in bothaxial end regions of at least one winding section of the current densityin each of a plurality of winding layers relative to a current densityin regions of the normal current density bordering the end regions, andmaximum axial pressures, acting on the wire, due to Lorentz forces inthe plurality of winding layers are held within allowable limits, saidwinding sections being configured so that in the end regions, thecurrent density is reduced by using rectangular wire having a width toheight relationship larger than the width to height relationship ofrectangular wire in the region of normal current density.